Titanium alloys are widely used in the aerospace industry because they provide a useful combination of low density, high strength, good toughness, good resistance to corrosion and oxidation, and the ability to retain these properties at temperatures up to about 950.degree. F. However, the nature of aerospace vehicles and propulsion systems, where unnecessary weight exacts a penalty in reduced payload or fuel economy, indicates that designers of such vehicles and systems often seek to load the materials used therein as near to their performance limits as is reasonably prudent. Doing so requires that aerospace materials be made with special care, including careful inspection, to ensure that sound material be used in such a vehicle or system.
Ultrasonic inspection is widely used to inspect nonferrous alloys for aerospace applications. The inspection may be done on semifinished mill products, such as billets, and again on forgings made from those mill products. The double inspection has generally been justified on the basis that the inspection of billets does not detect all of the material imperfections, even those which originate in the manufacture of ingots. However, if ultrasonic inspection of billets could be made more sensitive, it might be possible to omit the second inspection. Besides eliminating the cost of the second inspection operation, imperfections would be identified and removed from the manufacturing process before the cost of forging is incurred; thus, the concept of relying on ultrasonic inspection solely at the billet stage is economically attractive. All metals contain imperfections, which are inherent in their formation. Some imperfections are larger than others with small imperfections not being capable of detection. The smallest size capable of being detected is referred to as the minimum resolvable size.
Ultrasonic inspection is affected by two intrinsic characteristics of the material being inspected, namely, attenuation of the sound waves and reflection of the sound waves within the material itself. Attenuation is manifested as weakness of any signals generated in response to internal structure and imperfections in the material due to scattering of the signal. Reflections from internal structural features as grain boundaries are manifested as noise, which are sometimes referred to as "grass" in the electronic image of the ultrasonic inspection, while reflections from imperfections are frequently referred to as indications. Unfortunately, most titanium alloys suffer from both high attenuation and high noise levels. Consequently, it is difficult to distinguish valid ultrasonic reflections indicating the presence of a subsurface indication from noise, particularly in large diameter billets where the indication may be as much as seven or eight inches below the surface. In comparison, other alloy families, such as aluminum-, iron- and nickel-base alloys, permit much more sensitive ultrasonic inspection than do titanium alloys.
Several investigators have attempted to determine the cause for the high attenuation and high noise in ultrasonic inspection of titanium alloys, specifically Ti-6Al-4V (Ti-64), which is a widely used alpha-beta alloy. Billman and Rudolph ("Effects of Ti-6Al-4V Metallurgical Structure on Ultrasonic Response Characteristics," Titanium Science and Technology, edited by Jaffe and Burte, Vol. 1, (1973), pp. 693-70.5, Plenum Press, New York) evaluated ultrasonic inspection behavior of Ti-64, as such behavior is affected by macrostructure and microstructure, and by deformation below the beta transus. They pointed out the importance of refining the alpha platelets in the microstructure, which they were able to accomplish by beta recrystallization at 2000.degree. F. and by extensive deformation below the beta transus. The beta transus of a titanium alloy is that temperature above which the alpha phase does not exist at equilibrium conditions.
Allison, Russo, Seagle and Williams ("The Effect of Microstructure on the Ultrasonic Attenuation Characteristics of Ti-6Al-4V, "Titanium Science and Technology, edited by Lutjering, Zwicker and Bunk, Vol. 2, (1985), pp. 909-916, Deutsche Gesellschaft fur Metallkunde) showed the value of quenching from a temperature above the beta transus to reduce ultrasonic attenuation in Ti-64.
Granville and Taylor ("High Noise Levels during the Ultrasonic Testing of Titanium Alloys," British Journal of NDT, May, 1985, pp. 156-158) point out the importance of the shape and distribution of alpha particles in determining alloy behavior during ultrasonic inspection.
U.S. Pat. No. 3,470,034, the disclosure of which is incorporated herein by reference, teaches refinement of the microstructure by deformation starting above and ending below the beta transus as a means for reducing noise in ultrasonic inspection.
U.S. Pat. No. 3,489,617, the disclosure of which is incorporated herein by reference, teaches the deformation and beta recrystallization for breaking up alpha networks at prior beta grain boundaries to achieve a better combination of ductility and strength. Both of these patents are directed to Ti-64
As indicated hereinabove, prior work has been directed toward Ti-64 and related alloys; titanium alloys containing greater amounts of the beta stabilizing alloying elements have not received nearly as much attention. Titanium alloys . containing large amounts of beta stabilizing alloying elements are often identified as beta-stabilized alpha-beta alloys, or beta alloys.
The present invention is directed toward a need for improving the ultrasonic response of beta-stabilized alpha-beta and beta titanium alloys. Such alloys respond differently to thermal and deformation processing, and to ultrasonic inspection, than do the alpha alloys and alpha-beta alloys such as Ti-64. Therefore, the methods used to improve the response of alpha and alpha-beta alloys are not capable of providing similar improved response for the beta and beta-stabilized titanium alloys.